Real Life atmospheric limit

[PTNLemay]

I know that the parallel doesn't quite work, in KSP you have a very clear cut barrier at 70 Km, bellow which anything you leave in orbit will definitely come back down. Whereas in reality the atmospher is spread out in a very gradual fashion.

But I'm wondering if there's a general barrier based on that criteria, a point bellow which debris can't hang around for long because the atmospheric drag is just too much. In fact I think the ISS could almost fall under this criteria, since it does loose it's orbit slowly over time. But since it orbits 16 times a day and it takes over a year for it to drop 100 km, I think we can call it roughly stable. But yeah, is there a point in the atmosphere where debris can't survive for more than... 50 orbits? Or maybe 100 orbits?

(The real ISS's altitude over time: http://en.wikipedia.org/wiki/File:Internationale_Raumstation_Bahnh%C3%B6he_(dumb_version).png.)

Edited August 16, 2013 by PTNLemay

[astropapi1]

Anything below 100 km should do. And even then, something orbiting at 100 km wouldn't last long.

[legoheli]

Even satellites in geosynchronous orbits experience atmospheric drag as well as gravitational pulls of other bodies than Earth causing orbits to change over time. A stable orbit as in KSP does simply not exist in the real world.

Regarding debris, however, you'll have to take several things into factor:

How big is it?

How aerodynamic is it really?

How eccentric is the orbit?

Some of the spent rocket stages re-enter after just a few days - when COTS2 Dragon was "dead", it would re-enter automatically with no thrusters fired after 4 days, as would the spent 2nd stage of the Falcon 9 - and the 2nd stage of course did re-enter.

[DerekL1963]

All those questions are neatly summed up in one term: ballistic coefficient.